1. Technical Field
The present disclosure relates to a method for manufacturing an HEMT transistor. The disclosure also relates to a HEMT transistor.
2. Description of the Related Art
As it is well known, a high electron mobility transistor (HEMT) substantially is a field effect transistor including a channel region realized by a junction between two materials having different band gaps, i.e., a heterojunction. The HEMT transistor is then also known as heterostructure FET (HFET).
In particular, by employing two semiconductor materials with different band-gaps, an electron potential well is formed at the hetero-interface between the used materials. The electrons are confined in this potential well to form a two-dimensional electron gas.
The fundamental characteristic of a HEMT transistor is the conduction band offset between the materials which realize the barrier and channel layers, that is, the barrier layer has a higher conduction band while the channel layer has a lower conduction band. In this way, a potential well is formed which can contain a large number of electrons to form a two-dimensional electron gas channel at the hetero-interface due to this conduction band offset.
Wide-bandgap semiconductors are especially promising for realizing HEMT transistors, in particular gallium nitride and indium gallium. Devices incorporating indium generally show better high-frequency performance, while in recent years, devices incorporating gallium nitride have attracted attention due to their high-power performance.
High mobility two dimensional electron gas is generated using a heterojunction of a highly-doped wide-bandgap n-type donor-supply layer and a non-doped narrow-bandgap channel layer with no dopant impurities.
A widely used material combination is a thin layer of aluminum gallium nitride, AlGaN, being n-doped and a non-doped layer of gallium nitride, GaN.
In this case, electrons generated in the thin n-type AlGaN layer drop completely into the GaN layer to form a depleted AlGaN layer, because the so created heterojunction forms a quantum well (a steep canyon) in the conduction band on the GaN layer side where the electrons can move quickly without colliding with any impurities, the GaN layer being undoped, and from which they cannot escape. In this way, a very thin layer of highly mobile conducting electrons with very high concentration is created, giving the channel of the transistor so formed a very low resistivity and a high electron mobility.
HEMT transistors comprising an AlGaN/GaN heterojunction are devices that can operate at high power, at high speed and in a high temperature environment. The high performance of a device of this kind is achieved, for instance, by using depletion-mode (D-mode) HEMT transistors, in which the threshold voltages typically range between −4V and −8V.
Also known are the enhancement-mode (E-mode) HEMT transistors, which have a simple circuit design thanks to the elimination of the negative voltage supply and a low standby power dissipation during switching, being normally off, as described in the article to N. Ikeda entitled: “Normally-Off Operation Power AlGaN/GaN HFET”, Proc. Int. Symp. Power Semicond. Devices and ICs, pp. 369-372, 2004.
Various technologies for developing normally off devices which uses a Schottky junction gate or a MOS gate structure are also described in the article to H. Kambayashi entitled: “Normally-Off N-Channel GaN MOSFETs on Si Substrates Using an SAG Technique and Ion Implantation”, IEEE Electron Device Lett., vol. 28, no. 12, pp. 1077-1079, December 2007.
High-voltage AlGaN/GaN HEMT transistors have been so far manufactured using multiple field plates over dielectric passivation layers, the device breakdown voltage increasing with the addition of the field plates. In particular, the manufacturing of these devices has been performed using the well known stepper lithography, which has the advantages of a consolidated technique and allows to achieve a high throughput.
Alternatively, to guarantee high RF performance, E-Beam lithography may be used for aggressive critical dimensions. In this case, a very low-throughput is obtained, also for the fact that this technique is not a consolidated process.
Also known from the U.S. patent application published under No. 1 921 669 in the name of Cree, Inc. is a GaN based HEMT with buried field plates. Moreover, the European patent application published under No. 1 965 433 still in the name of Cree, Inc. describes a high voltage GaN transistor having multiple field plates realized on different spacers. The transistor structures described and illustrated in these patent applications comprise an insulating layer which has spacer-like structures.
An E-mode AlGaN/GaN HEMT transistor of the known type is schematically shown in FIG. 1, globally indicated with 100. The E-mode HEMT transistor 100 in particular comprises a p-doped GaN buffer layer and an over-recessed gate that have been realized by etching the barrier layer being under a gate metal structure of the E-mode HEMT transistor 100, in this way increasing the threshold voltage due to depletion effect and reducing the buffer leakage current, as described in the article to Ki-Sik Im entitled: “Normally Off GaN MOSFET Based on AlGaN/GaN Heterostructure With Extremely High 2DEG Density Grown on Si Substrate”, IEEE Electron Device Lett., Vol. 31, no. 3, March 2010 and in the article to Dong-Seok Kim entitled: “Normally-Off Operation of Al2O3/GaN MOSFET Based on AlGaN7GaN Heterostructure With p-GaN Buffer Layer”, Proceedings of the 22nd International Symposium on Power Semiconductor Devices & ICs, Hiroshima, NM-P5.
With reference of FIG. 1, the E-mode HEMT transistor 100 comprises a substrate 1, made of silicon or silicon carbide or sapphire, for instance, covered by a buffer layer 2, in turn covered by the series of a bottom transition layer 3 and a top transition layer 4. Other type of material may be used for realizing the substrate 1.
On the top transition layer 4, the E-mode HEMT transistor 100 comprises a highly resistive layer 5 made of n-type gallium nitride, GaN as well as a first and a second portion, 6A and 6B respectively, of a layer made of aluminum gallium nitride, AlGaN.
The E-mode HEMT transistor 100 further comprises a source structure and a drain structure, 8S and 8D respectively, realized on the first and second portions, 6A and 6B respectively, of the AlGaN layer, as well as a gate structure 8G, realized over the GaN layer 5 and separated therefrom by an insulator layer 7, being a dielectric with high-K like Al2O3., HfO2, ScO2, etc. In particular, the gate structure 8G is realized in correspondence with a recessed portion 5RP of the GaN layer 5.
As above explained, at the interfaces between the portions 6A and 6B of the AlGaN layer and the GaN layer 5, a first and a second two-dimensional electron gas channel are obtained, indicated in FIG. 1 with 9A and 9B, respectively.
The dimension of the gate structure is still a critical parameter influencing the maximum working frequency of the HEMT transistor itself.